When I "Mark as Reviewed" an action from a list of single actions, the whole list gets marked as reviewed. Shouldn't this function work on an action-by-action basis when it comes to lists of single actions? I think it would make the function much more useful...

I initially had the same reaction, but then I thought to myself, What is the point of a review? It's to ensure that I'm aware of a project (reinforcement through repetition) and have identified the next action that I can take on it.

Since a single action is its own next (and last) action, there's no further task to identify in the review. And if you have scanned over the entire single action list, you've reinforced your memory of them. I don't see what purpose would be served by making the action, as opposed to the list, something that could be marked as reviewed.

If you have some single actions that you would like to mark reviewed, independently of the others, perhaps you should promote them to projects and then break them down into more concrete actions.

Could you give us a specific example or two of how this feature would help?